This invention relates to an optical encoder in which a grating plate and a scanning grating plate are located between a light source and a light receiving element for relative movement to each other to detect variations in luminous energy transmitted through the gratings due to relative displacement of the main grating plate and the scanning grating plate, to thereby measure the amount of the relative movement of the grating plates.
The above light-transmissive type optical encoder generally comprises a main grating plate and a scanning grating plate both with equally spaced gratings slits, which are located in parallel to each other between the light source and the light-receiving element for relative movement to each other. The light emitted from the light source is subject to changes due to relative displacement of the gratings formed on both the main grating plate and the scanning grating plate before reaching the light receiving element. The light receiving element then produces an electric signal of an approximate sine wave whose cycle coincides with the grating pitch in dependence on relative displacement of the gratings of the main and scanning grating plates. The measured value of relative movement of gratings the is given in the unit of grating pitch by counting the peak cycles of such an electric signal.
A rotary optical encoder of the above type, for example, is provided, between the light source and the light receiving element both of which are fixed in position, with a rotary grating plate (corresponding to the main grating plate mentioned above) with radially extending spaced grating slits and one or more scanning grating plate located adjacent the rotary grating plate in parallel therewith, which is inhibited from movement relative to the light source and the light receiving element.
As the rotary grating plate rotates, the luminous energy passed through the gratings of both grating plates varies in a cycle of grating pitch in response to relative positional displacement of these gratings. Varying luminous energy thus reaches the light receiving element which then produces an electric signal varying generally in the sine wave form with its cycle corresponding to the grating pitch. Such a signal varying in sine waveform is processed by a signal processing circuit to generate pulse signals. The angular position of the rotary grating plate is thus given in the unit of grating pitch by computing the pulse signals.
In such an optical encoder, measuring accuracy rises as the number of pulse signals produced for each relative movement of the main grating plate with respect to the scanning grating plate is increased. That is, the narrower the grating pitch on the grating plates, the better the acuracy in measuring the relative movement of the main and scanning grating plates would be.
It is also possible to measure the relative movement of the main and scanning grating plates with a higher accuracy than a given grating pitch by using pulse signals that are more fractionized than the grating pitch, utilizing a phase division technique for the sine wave signal detected by the light receiving element. To achieve this, the signal variation detected by the light receiving element (i.e., the variation in luminous energy reaching the light receiving element due to relative movement of the gratings) must be a correct sine wave in the cycle of the gratings pitch. With any waveform deviated from the correct sine wave, accurate phase division could not take place, resulting in a lowered accuracy in measuring the variation.
While a light beam can substantially be considered as linearly advancing when it passes through gratings with a rough pitch, the gratings begin to function as diffraction gratings so that the light passed through them will be subject to a diffraction effect, as the grating pitch is made finer to improve the measuring accuracy. In order to maintain a required measuring accuracy with no effect of such diffraction, the distance between the main grating plate and the scanning grating plate (grating distance) must be maintained within a very limited range of allowance. Specifically, the light passed through the grating plate is subject to a diffraction effect which is known as Fresnel diffraciton in terms of optics to provide a diffraction light having more than one peak with respect to the distance from the main grating plate (light and darkness alternately appear at the same pitch as the grating of the main grating plate depending on the distance from the main grating plate). The location of such peaks depends on the grating pitch and the wavelength of the light passed therethrough. The position of the scanning grating plate with respect to the main grating plate (distance between the main grating plate and the scanning grating plate) is preferably made coincident with the location of the peaks of the Fresnel diffracion light formed by the grating of the main grating plate and is usually set at the first peak of the Fresnel diffraction light.
With the scanning grating plate located at the first peak accordingly, the following difficulties may be encountered, if there is a locational error of the grating plate.
With an increased grating distance, the direct current component of a detected signal rises to lower contrast in signal, resulting in a degraded S/N ratio. This adversely affects the accuracy in converting the detected signal to pulse signals and therefore the entire measuring accuracy.
Too small a grating distance, on the other hand, will cause a harmonic distortion of the detected signal to increase (the waveform of the detected signal approximates a rectangular wave rather than a sine wave), so that accuracy in phase division is degraded to again lower the measuring accuracy as a whole.
Thus, in order to obtain a well-defined sine waveform, the grating distance must be maintained within a very narrow range of values. Also, there may be errors in measurement due to variation in surface evenness as well as varying grating distance due to relative movement of the main grating plate and scanning grating plate. Consequently, significant precision is required for fabricating components and their assembly and adjustment, resulting in higher manufacturing costs. Particularly in a rotary encoder, even eccentricity of the rotary grating plate causes measuring errors, requiring a far greater precision in manufacturing and adjusting its rotary mechanism.
Also, the grating distance must be diminished with a finer grating pitch (e.g., assuming grating pitch to be 8 .mu.m and the wavelength .lambda. of light to be 0.95 .mu.m, the grating distance should be d=60 .mu.m). Because dust allowed into such a narrow grating distance may damage the grating surface to deteriorate the signal, a necessary dust protecting means further pushes up the manufacturing cost.
Furthermore, since the Fresnel diffraction image is formed periodically for each of the grating pitches of both grating plates, the cycle of the sine wave signal fluctuates depending on the location of the main grating plate, if there is an error in grating slit width or grating pitch in each grating plate. This also makes it difficult to provide uniform measuring accuracy.